“Maybe it’s time for world-stock funds, rather than ones that focus separately on the U.S. and overseas,” advises a story in The Wall Street Journal. It’s a good idea—up to a point. The strategy of using core funds, perhaps even a super core for the entire asset allocation process, has merit. But there’s a danger of going too far. The problem is that if you put too much in a core fund, rebalancing opportunities are limited if not short-circuited completely, depending on the core fund you hold and how much it represents of your allocation in that asset class.